Certain photodiodes have two-electrode radiation-sensitive junctions formed in semiconductor material. Light, which illuminates the junction, creates charge carriers. Reverse current varies with illumination. Photodiodes can be photovoltaic or photoconductive devices, used for detection of optical power and subsequent conversion to electrical power.
Operationally, photodiodes absorb photons or charged particles, facilitating detection of incident optical power and generating current proportional to the incident power, thus converting the incident optical power to electrical power. Light-induced current of the photodiode corresponds to the signal while “dark” current represents noise. “Dark” current is that current that is not induced by light, or that is present in the absence of light. Photodiodes process signals by using the magnitude of the signal-to-noise ratio.
Photodiodes are typically characterized by certain parameters, such as, among others, electrical characteristics, optical characteristics, current characteristics, voltage characteristics, and noise. Electrical characteristics predominantly comprise shunt resistance, series resistance, junction capacitance, rise or fall time and/or frequency response. Optical characteristics comprise responsivity, quantum efficiency, non-uniformity, and/or non-linearity. Photodiode noise may comprise, among others, thermal noise, quantum, photon or shot noise, and/or flicker noise.
Conventionally, in an effort to increase the signal to noise ratio and enhance the contrast of the signal, it is desirable to increase the light-induced current of photodiodes. Thus, photodiode sensitivity is enhanced while the overall quality of the photodiode is improved. Photodiode sensitivity is crucial in low-level light applications and is typically quantified by a parameter referred to as noise equivalent power (NEP), which is defined as the optical power that produces a signal-to-noise ratio of one at the detector output. NEP is usually specified at a given wavelength over a frequency bandwidth.
Essentially active solid-state semiconductor devices, and in particular, silicon photodiodes, are among the most popular photodetectors having a sufficiently high performance over a wide wavelength range with ease of use. Silicon photodiodes are sensitive to light in the wide spectral range, approximately 200 *10−9 m to 1100*10−9 m, extending from deep ultraviolet through visible to near infrared. Silicon photodiodes, by using their ability to detect the presence or absence of minute light intensities, facilitate the extremely precise measurement of these minute light intensities upon appropriate calibration. For example, appropriately calibrated silicon photodiodes detect and measure light intensities varying over a wide range, from very minute light intensities of below 10−13 watts/cm2 to high intensities above 10−3 watts/cm2.
Conventional photodiodes, as described above, still present numerous problems in their use today. Many of the resources associated with the use of conventional photodiodes, including capital, time, and manpower are wasted in resolving some of the numerous problems including controlling (also referred to as modifying, managing, reducing or eliminating) photodiode leakage current, noise, and stray light. In addition, there are secondary problems that pose problems in conventional photodiode use. These include, but are not limited to, determination and/or selection of apt active area specifications and controlling the higher-wavelength infrared response.
A particularly serious problem associated with conventional photodiodes is leakage current. Leakage current is a major source of signal offset and noise in applications operating in current mode. More specifically, leakage current flows through the photodiode when it is in a “dark” state, or in the absence of light at a given reverse bias voltage applied across the junction. Leakage current is specified at a particular value of reverse applied voltage. For example, reverse bias voltage may be as low as 10 mV or as high as 50 V, whereas the “dark” currents may vary from pA to μA depending upon the junction area and the process used. Leakage current is temperature dependent; thus, an increase in temperature and reverse bias results in an increase in leakage or dark current. A general rule is that the dark current will approximately double for every 10° C. increase in ambient temperature. It should be noted, however, that specific diode types can vary considerably from this relationship. For example, it is possible that leakage or dark current will approximately double for every 6° C. increase in temperature.
Various approaches have been used in the prior art to reduce, eliminate or control leakage current. For example, U.S. Pat. No. 4,904,861, assigned to Agilent Technologies, Inc., discloses “an optical encoder comprising: a plurality of active photodiodes in an array on a semiconductor chip; a code member having alternating areas for alternately illuminating and not illuminating the active photodiodes in response to movement of the code member; means connected to the active photodiodes for measuring current from the active photodiodes; and sufficient inactive photodiode area on the semiconductor chip at each end of the array of active photodiodes to make the leakage current to each end active photodiode of the array substantially equal to the leakage current to an active photodiode remote from an end of the array”. Similarly, U.S. Pat. No. 4,998,013, also assigned to Agilent Technologies, Inc. discloses “means for shielding a photodiode from leakage current comprising: at least one active photodiode on a semiconductor chip; means for measuring current from the active photodiode; a shielding area having a photodiode junction substantially surrounding the active photodiode; and means for biasing the shielding area photodiode junction with either zero bias or reverse bias.”
U.S. Pat. No. 6,670,258, assigned to Digirad Corporation, discloses “a method of fabricating a low-leakage current photodiode array comprising: defining frontside structures for a photodiode on a front side of a substrate; forming a heavily-doped gettering layer on a back surface of the substrate; carrying out a gettering process on the substrate to transport undesired components from the substrate to said gettering layer, and to form another layer in addition to said gettering layer, which is a heavily-doped, conductive, crystalline layer within the substrate; after said gettering process, removing the entire gettering layer; and after said removing, thinning the heavily-doped, conductive, crystalline layer within the substrate to create a native optically transparent, conductive bias electrode layer”.
Similarly, U.S. Pat. No. 6,734,416, also assigned to Digirad Corporation, discloses “a low-leakage current photodiode array comprising: a substrate having a front side and a back side; a plurality of gate regions formed near the front side of the substrate; a backside layer formed within the substrate, near the back side of the substrate, the backside layer having a thickness of approximately 0.25 to 1.0 micrometers and having a sheet resistivity of approximately 50 to 1000 Ohm per square.”
U.S. Pat. No. 6,569,700, assigned to United Microelectronics Corporation in Taiwan, discloses “1. A method of reducing leakage current of a photodiode on a semiconductor wafer, the surface of the semiconductor wafer comprising a p-type substrate, a photosensing area for forming a photosensor of the photodiode, and a shallow trench positioned in the substrate surrounding the photosensing area, the method comprising: forming a doped polysilicon layer containing p-type dopants in the shallow trench; using a thermal process to cause the p-type dopants in the doped polysilicon layer to diffuse into portions of the p-type substrate that surround a bottom of the shallow trench and walls of the shallow trench; removing the doped polysilicon layer; filling an insulator into the shallow trench to form a shallow trench isolation (STI) structure; performing a first ion implantation process to form a first n-type doped region in the photosensing area; and performing a second ion implantation process to form a second n-type doped region in the photosensing area.”
Also, U.S. Pat. No. 6,504,158, assigned to General Electric Company, discloses “a method of reducing leakage current in an imaging apparatus, including: providing a substrate with at least one radiation-sensitive imaging region therein; forming a guard region in the substrate at or immediately adjacent a cut edge of the substrate to reduce leakage current reaching the at least one radiation-sensitive imaging region from the cut edge when the imaging apparatus is in use; and electrically reverse biasing the at least one radiation-sensitive imaging region and the guard region relative to the substrate.”
In addition to leakage current, noise is often a limiting factor for the performance of any device or system. In almost every area of measurement, the limit to the detectability of signals is set by noise, or unwanted signals that obscure the desired signal. As described above, the NEP is used to quantify detector noise. Noise issues generally have an important effect on device or system cost. For example, the reduction of quantum or shot noise influence in interferometers for gravitational wave detection greatly lessens required interferometer arm length. Conventional photodiodes are particularly sensitive to noise issues. Like other types of light sensors, the lower limits of light detection for photodiodes are determined by the noise characteristics of the device.
As described above, the typical noise components in photodiodes include thermal noise; quantum or shot noise; and flicker noise in that order. These noise components collectively contribute to the total noise in the photodiode. Thermal noise, or Johnson noise, is inversely related to the value of the shunt resistance of photodiode and tends to be the dominant noise component when the diode is operated under zero applied reverse bias conditions. Shot noise is dependent upon the leakage or dark current of photodiode and is generated by random fluctuations of current flowing through the device, which may be either dark current or photocurrent. Shot noise tends to dominate when the photodiode is used in photoconductive mode where an external reverse bias is applied across the device. As an example, detector noise generated by a planar diffused photodiode operating in the reverse bias mode is a combination of both shot noise and thermal noise. Flicker noise, unlike thermal or shot noise, bears an inverse relationship to spectral density. Flicker noise may dominate when the bandwidth of interest contains frequencies less than 1 kHz.
In addition to leakage current and noise issues, photodiodes are susceptible to absorption of unwanted or stray light. Stray light is typically that light which propagates through a path other than that which is intended (thus reaching the detector) and excludes the wavelength of interest or an erroneous wavelength. Stray light activates a signal at the detector element. Some of the recognized sources of stray light include ambient light, scattering light from imperfect optical components and/or reflections of non-optical components. The ambient or stray light is assumed to be stationary or slowly fluctuating. Stationary ambient light engenders shot noise, owing to the statistical nature of the production of photocarriers upon impinging on the photodetector. In certain circumstances, the photodiode is exposed to ambient or stray light thus resulting in a high level of noise. Additionally, stray light influences the signal-to-noise ratio (S/N or SNR) and introduces non-linearity.
While photodiodes have a wide viewing angle, many applications require the detection of desired light and concurrent rejection of stray light. Stray light absorption significantly contributes to the total photocurrent delivered by a small active area photodiode and renders it not usable. For example, photodiodes typically used in medical applications need to deliver a precise ratio of photocurrents illuminated from 660 nm and 880 nm. It is possible that an incorrect ratio of photocurrents will be delivered due to the absorption or collection of stray light by the photodiode.
Some attempts have been made in the prior art to correct for the effects of stray light. For example, U.S. Pat. No. 5,869,834, assigned to Sitek Electro Optics, discloses “a position-sensitive photodetector for measuring the position of an incident light beam having energy, comprising: a semiconductor wafer having a doped active surface and a pair of ends; a resistive layer on the active surface of the wafer; two opposed electrodes on each end of said active surface; a stray-light area arranged externally around the active surface, said stray-light area for measuring stray-light contacting said photodetector and for preventing said stray light from affecting a position measuring signal of said incident light beam, said stray light area comprising a doped area containing a p-n junction and a grounding electrode attached thereto, the signal from said stray light area being grounded; a first inactive area externally surrounding said stray-light area and a second inactive area disposed between said active area and said stray-light area, each inactive area connected to said same ground as said stray-light area.”
Also, U.S. Pat. No. 6,546,171, assigned to NEC Corporation, discloses “a structure for shielding a stray light in an optical waveguide module, comprising: an optical waveguide substrate supported in an optical waveguide module package; a laser diode (LD) which is mounted on said optical waveguide substrate and emits a first signal light; an optical waveguide which is formed on said optical waveguide substrate and propagates a part of said first signal light; a wavelength divisional multiplexing (WDM, hereinafter) filter which is formed on said optical waveguide substrate; an optical fiber which is situated on said optical waveguide substrate and transmits said part of said first signal light to an optical transmission line; a photodiode (PD, hereinafter) for receiving a second signal light which is propagated through said optical transmission line and transmitted through said WDM filter via said optical waveguide; and a metallic layer which is evaporated on a surface of said WDM filter and provided with a pin hole for transmitting said second signal light to said PD via said WDM filter, wherein a remaining part of said first signal light (a stray light, hereinafter) which is not optically coupled with said optical waveguide is reflected by said metallic layer so that said stray light is prevented from being coupled with said PD.”
As discussed earlier, secondary issues also contribute to dark noise and other noise sources that impact photodiode sensitivity. These include primarily determination and/or selection of apt active area specifications (geometry and dimensions), response speed, quantum efficiency at the wavelength of interest, response linearity, and spatial uniformity of response, among others.
While methods and systems of the prior art described above may improve performance, they do not effectively control leakage current, which is equally important to high-performance as well as low-standby and low-power operation devices. Thus, there is still a need for photodiodes with controlled leakage current. Additionally, there is need for low-voltage, low-power designs vis-à-vis photodiodes with aggressive techniques for controlling leakage current. In addition, in spite of the attempts in the prior art, as described above, to reduce or eliminate the effects of stray light absorption with respect to photodiodes, no attempt has been made to design and fabricate photodiodes with special structures to completely avoid stray light.
Thus, there is a need for the design and implementation of methods and apparatuses which completely avoid stray light absorption, reduce or eliminate noise, and control leakage current while lessening the effects of these noise contributing factors. More specifically, what is needed is a photodiode design and fabrication method intended for applications wherein a small active area is employed and wherein a complete avoidance of stray light is of greater importance.